1. Field of the Invention
This invention relates to a method for improving the flow of acrylic molding powders.
2. Description of the Prior Art
In the acrylic molding powder art, it is generally desirable for the acrylic molding polymers to have as low a melt viscosity as possible without a sacrifice in other important properties such as heat distortion, impact strength, and toughness. A low melt viscosity facilitates molding because it permits the use of low melt temperatures without excessively high mold filling pressures. For a given softening temperature, a lower melt viscosity permits faster cooling and therefore higher molded part production rates. One prior method of reducing polymer melt viscosity was to lower the molecular weight of the polymer; however, below a certain molecular weight level strength and toughness of the molding polymer are adversely affected. Another prior way to reduce melt viscosity was to blend the molding polymer with a low viscosity additive or plasticizer. See, for example, U.S. Pat. No. 3,655,829 of Apr. 11, 1972 to Ronzoni et al., and U.S. Pat. No. 3,809,667 of May 7, 1974 to Coaker et al. Also see West German Pat. No. 2,264,628 of July 12, 1973 corresponding to U.S. Ser. No. 215,114 of Jan. 3, 1972. However, these prior methods cause a loss in heat distortion, creep resistance, clarity, and/or toughness. An alternative method to reduce viscosity is to lower the overall glass temperature by copolymerizing with the monomer of a polymer having a relatively low glass temperature, but this also reduces heat distortion resistance. Others have attempted to improve the melt flow rate of thermoplastic polymers with such modifiers as an alpha-methyl benzyl alcohol diester of carbonic, phthalic, or (C.sub.2 -C.sub.10) normal aliphatic saturated dicarboxylic acid (Showa Denko KK, Derwent Abstract 6436T corresponding to Japanese Pat. No. 7238540), ethylene glycol fatty acid diester mixed with pentaerythritrol fatty acid tetraester and alkaline earth metal soap (Nippon Oils and Fats Company, Ltd., Derwent Abstract 67326U corresponding to Japanese Pat. No. 7335697), a higher fatty acid amide or xylene diamine added during polymerization (Derwent 67832S corresponding to Japanese Pat. No. 7136458), or xylene diamine stearic acid amide (Derwent 57831S corresponding to Japanese Pat. No. 7135457).
Coover et al, U.S. Pat. No. 2,887,464, teach modification of copolymers of methyl methacrylate and .alpha.-methyl styrene with a lesser proportion of an acrylic or methacrylic ester polymer or with a lesser proportion of a butadiene-1,3 polymer. Coover et al do not suggest modification of all acrylic molding powder polymers.
None of the prior systems allows for increased melt flow of acrylic molding powder without significantly diminishing the heat distortion strength and toughness.